Activated carbon materials have found use in a variety of applications such as gas and liquid adsorption applications, for example radon testing, gas masks, sugar refining, municipal water treatment, removal of microbiological agents, adsorption of volatile organic compounds, separation of colloidal or dissolved organic substances, etc.
The predominant commercial use for activated carbon is in the form of granules. While activated carbon in the form of granules can perform the desired adsorption for many applications, there are some applications in which the granules have drawbacks. In some cases back pressure of a packed bed of granules is a problem. Some applications can result in considerable wear of the granules by attrition, causing loss of material or bed packing. Furthermore, the fines which are generated as a result of attrition can block the flow path.
Another approach is to use an extruded activated carbon in the form of a cellular structure such as a honeycomb. The honeycomb can readily be shaped by extruding fine powders of activated carbon with suitable binders. Such a shape allows for ease of flow of the gases through the honeycomb with little back pressure. Also, the geometry can be such as to allow easy access of the gases to all of the carbon for adsorption of the species to be removed. Also, since the honeycomb is a solid piece, there should be little or no wear or attrition of the carbon.
In order to form an activated carbon honeycomb by extrusion, the carbon must be in the form of a fine powder. This can then be mixed with a liquid such as water and suitable plasticizers and binders. This plasticized mixture is then extruded through a die into the honeycomb shape, and dried.
These bodies sometimes suffer from low strength both in the as-extruded state and in the as-dried state. They can also develop cracks during the drying procedure. This is especially evident in the larger bodies due to differential shrinkage which occurs because of loss of moisture between the outer surfaces and the interior of the body. Also, they suffer from lack of water resistance which results in their deterioration when exposed to water.
Clays and resins have been used as binders in carbon mixtures to impart strength to the carbon body formed therefrom.
U.S. Pat. Nos. 3,825,460, 3,922,412, and 4,399,052, GB patent application 2,235,684A, Japanese patent application publication 49-115110 (1974), and 55-167118 (1980) relate to various methods of making carbon bodies using phenolic binders. Some of these references teach adding resin before activating the carbon. This procedure is relatively complicated as it involves carbonization of the phenolic resin binder, thereby destroying its properties.
Japanese patent application publication No. 55-167118 (1980) relates to a manufacturing method of activated charcoal formed material having water soluble organic binder, powder shape thermosetting resin, and powder shape activated charcoal, which are kneaded with water, formed to the desired shape and then heat-treated for hardening.
U.S. Pat. No. 3,634,569 relates to a method of forming a high density graphite structure by preparing a slurry of a thermosetting phenolic resin binder with a liquid dispersant, drying the slurry to drive off the dispersant, distributing the graphite and resin in a mold and forming the structure under pressure at a temperature of up to about 400.degree. F.
It is highly desirable therefore, to improve the strength of the extruded honeycomb both in the extruded state for further processing and handling and also after drying to improve performance. It is also desirable to improve the drying procedures so that the bodies are produced crack-free. Furthermore, it is desirable to produce such bodies which maintain their structural integrity when in contact with water.
The present invention provides such improved bodies and a method for making them.